The present invention relates to systems and methods for controlling the ratio of the mixture of recirculated exhaust gas and intake air in a compression-ignition engine utilizing a turbocharger, and, in particular, a system and method for determining the flow rate of the recirculated exhaust gas.
In compression-ignition engines, such as heavy-duty diesel engines, the intake air is typically cooled and compressed, typically by using a turbocharger, to provide increased power density for the engine. Added flexibility in the compression of the intake air over a conventional turbocharger is often achieved by using a variable geometry turbocharger which may be controlled by the engine""s electronic control module (xe2x80x9cECMxe2x80x9d) to supply varying amounts of turbo boost pressure to the engine, depending on various operating conditions. One system for controlling an engine having a VGT is disclosed in U.S. Pat. No. 6,000,221, issued to Church et al. on Dec. 14, 1999.
One important objective for compression-ignition engine designers is to reduce NOX emissions, while minimizing the negative impact on engine fuel economy and durability.
It is therefore an object of the present invention to provide a system and method for reducing NOX emissions in a compression-ignition engine employing a turbocharger.
It is another object of the present invention to provide a system and method for measuring the flow rate of exhaust gas which is recirculated for combination with intake air in a compression-ignition engine.
In carrying out the above objects and other objects and features of the present invention, a system and method are provided for measuring the flow rate of recirculated exhaust gas in a compression-ignition engine including a plurality of engine sensors having outputs indicative of current engine conditions and a turbocharger. The system includes an exhaust gas recirculation (EGR) valve mounted in the exhaust pipe upstream of the turbocharger for diverting a selectable portion of the exhaust gas for recirculation and combination with the charge air, one or more sensors for sensing current conditions of the recirculated exhaust gas, including temperature and pressure, one or more sensors for sensing current conditions of the intake air, and control logic for determining the flow rate of the recirculated exhaust gas as a function of the sensed conditions.
In one embodiment, the system includes an obstruction in the flow path of the recirculated exhaust gas, a temperature sensor mounted for sensing the temperature of the recirculated exhaust gas, and a differential pressure sensor including a first pressure tap located for sensing the pressure of the recirculated exhaust gas upstream of the obstruction and a second pressure tap located for sensing the pressure of the recirculated exhaust gas downstream of the obstruction, and wherein the control logic includes logic for determining the flow rate of the exhaust gas as a function of the differential pressure drop across the obstruction and the exhaust gas temperature.
In another embodiment, the system includes a first temperature sensor mounted for sensing the temperature of the recirculated exhaust gas, a second temperature sensor mounted for sensing the temperature of the charge air, and a third temperature sensor mounted for sensing the temperature of the mixture of charged air and recirculated exhaust gas, and wherein the control logic includes logic for determining the flow rate of the recirculated exhaust gas as a function of the temperatures sensed by the first sensor, the second sensor, and the third sensor.
One embodiment of the system employs a thin plate obstructor which defines a relatively small diameter orifice in the exhaust gas recirculation pipe, thereby creating a relatively high pressure drop as the gas flows through the orifice. The thickness of the obstructor preferably ranges from about to 0.03 to about 0.08 pipe diameters, and most preferably is about 0.05 pipe diameters. The orifice defined by the obstructor is a circular opening having a diameter between about 60 and 80 percent of the pipe diameter, and most preferably about 60 percent of the pipe diameter. In one embodiment, the edge of the plate defining the orifice is beveled to achieve a sharper edge on the orifice plate, to thereby reduce diesel particulate deposits on the edge defining the critical diameter of the obstructor. It will be appreciated that the embodiment of the present invention which utilizes this thin-wall, sharp-edged obstructor offers relatively greater accuracy over the sensor life, since the thin wall and sharp edge design of the obstructor minimizes the amount of diesel particulate deposits on the edges of the orifice obstructor which define the orifice. Such deposits would, over time, reduce the effective orifice area and, thereby, reduce the system""s accuracy since, as is shown hereinafter, the EGR flow rate determination utilizes constants which are calibrated for the specific geometry.
In the differential pressure embodiment, the flow rate of the recirculated exhaust gas is determined from the voltage input from the differential pressure sensor, and from the sensed recirculated exhaust gas temperature, according to the following relation:
EGR Flow Rate (kg/min)=(EGR Gas Density/Density Correction)a*b*(Differential Pressure Drop, kPa)c 
where the Density Correction, a, b and c are each calibratable constants for a particular orifice design.
One advantage of employing the embodiment of the present invention that determines the flow rate as a function of the differential pressure drop across an obstruction is that the currently available pressure sensors provide relatively more accurate readings in a relatively faster response time than other sensors. Thus, the EGR can be reliably determined even during transient engine operation conditions.
The embodiment of the present invention which employs sensed temperature differential utilizes sensor inputs from each of the charge air temperature sensor, recirculated exhaust gas temperature sensor, and charge air/recirculated exhaust mixture temperature sensor, determines the recirculated exhaust gas flow ratio (EGR %) according to the following relationship:
The recirculated exhaust gas flow ratio       EGR    ⁢          xe2x80x83        ⁢    %    =                              m          .                egr                              m          .                air              =                            T          mixture                -                  T          air                                      T          egr                -                  T          mixture                    
It will be appreciated that one advantage of employing this embodiment of the present invention is that the system is non-intrusive to exhaust gas recirculation flow and results in a nearly non-existent pressure drop in the system.
In another embodiment of the present invention employs a differential pressure system of the type described above in conjunction with a differential temperature system so that, during steady state operation of the engine, the differential temperature system can be used to cross calibrate the differential pressure system.
The measuring system may be integrated with an engine control module (ECM) to provide an accurate EGR flow measurement as input to the ECM which can be used as feedback for the EGR valve, and/or the VGT controller to adjust the EGR valve and/or VGT vane positions and, consequently, control the rate of exhaust gas recirculation in a closed loop.
It will thus be appreciated that the system of the present invention allows for an accurate EGR flow measurement, thereby providing closed-loop controller feedback and input by which suitable control logic can detect a malfunction or tampering with the EGR flow circuit.
The above objects and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.